The invention relates to lithographic patterning of a plurality of identical structures, in particular large areas of periodic nanostructures, onto a target substrate.
One application of periodic nanostructures is the fabrication of near infrared (IR) mesh filter arrays. Photovoltaic energy conversion generally has poor efficiency with thermalxe2x80x94i.e., non-solarxe2x80x94energy sources due to the incongruity between the very broad photo-emission spectrum of thermal radiators with the narrow energy band of photovoltaic conversion. A solution to this problem has been described by Home et al. in U.S. Pat. No. 5,611,870. In that approach the photovoltaic cell is coupled with an infrared bandpass filter which transmits only those photons that can be efficiently converted to electricity by the photocells and reflects those with either shorter and longer wavelengths back into the source where they are reabsorbed. As infrared bandpass filter a metal-mesh filter array consisting of cross-shaped openings in a thin gold film is used; the cross-shaped openings have a length of 450 nm and an arm width of only 50-80 nm. The use of these filters is expected to increase the efficiency of GaSb photovoltaic cells from less than 1% to near 30% for operation with a 1500 K black-body radiator.
The challenge in forming patterns like the cross-shaped openings of the IR filter array lies in the resolution needed to reproduce the fine comers at the center of the cross. M. D. Morgan et al., in J. Vac. Sci. Technol. B 14 (6), 1996, pp.3903-3906, discuss the fabrication of IR filters using electron-beam lithography (EBL) and ion beam proximity printing (IBP) techniques, which were prepared with equivalent spectral response. The EBL approach uses a finely focused, high-energy beam of electrons to expose resist on a substrate. Deflectors are used to scan the beam across the substrate and so write the desired pattern with high accuracy, but the serial nature of this approach makes the fabrication process extremely time consuming and expensive.
FIG. 1 shows the principle of IBP patterning. Sensitive material 101, such as a photo-resist covering a substrate 102 which is to be structured according to the pattern defined in a pattern mask 103, is exposed by transmitted beamlets 104 that are formed when the mask 103 is illuminated by a broad beam 114 of light ions which pass through the transparent regions 105 to expose the underlying resist. In IBP, the mask 103 is positioned in proximity to the substrate 102; the distance G between the mask and the substrate is small, e.g. a few mm or less, and depends on the optical properties of the ion beam system. As illustrated in FIG. 1, the transparent regions 105 are typically openings. The openings of the stencil mask are, for instance, openings corresponding to the desired pattern, and the mask 103 is typically realized as a silicon membrane with etched openings. The parallel printing technique realized with the help of lithography masks replicates the pattern in the mask with a single exposure and so the cost to replicate a mask is effectively independent of array size up to a maximum determined by the size of the beam, which is typically 2 to 8 inches in diameter. Lithographic patterning methods, including printing and projection techniques, as well as lithographic devices using electron or ion beams are discussed, for instance, by H. Koops in xe2x80x98Electron beam projection techniquesxe2x80x99, Chapter 3 of xe2x80x98Fine Line Lithographyxe2x80x99, Ed. R. Newman, North-Holland, 1980, pp. 264-282. Electrons and in particular ions have the advantage of very low particle wavelengthsxe2x80x94far below the nanometer rangexe2x80x94which allow of very good imaging properties, as e.g. discussed by Rainer Kaesmaier and Hans Loschner in xe2x80x98Overview of the Ion Projection Lithography European MEDEA and International Programxe2x80x99, Proceedings SPIE, Vol. 3997, Emerging Lithography Technologies IV, 2000. Lithographic patterning using stencil masks is not restricted to particle beam systems, but also possible with lithography systems based on photons, like EUV (Extreme UV) or X-ray lithography. Also instead of transmission masks, reflection masks can be used, in particular in connection with EUV systems; in this case the mask has regions of higher reflectivity in place of transparent regions.
The stencil masks for IR filter arrays are conventionally fabricated using EBL to define the mask structures on a thin silicon membrane substrate. The high cost of the EBL process makes the fabrication of large-area IBP masksxe2x80x94i.e., greater than 1 cm2xe2x80x94uneconomical. To overcome this limitation, one can take advantage of the periodicity of the pattern to print step-and-repeat copies of a small mask onto a second mask substrate to form a second-generation replica with a much larger area. It would be desirable to repeat this replication process to generate subsequently larger generations of the original mask; however, in practice it proves difficult to maintain the fidelity of the original mask structure in even the first copy. This difficulty is due to the inherent blur of an ion beam system. Because of the blur the ion beam system acts as a low-pass spatial filter that attenuates the high-frequency information of a pattern of the original mask when imaged onto the substrate (e.g., a secondary mask).
The blur attenuates the high-frequency information that describes the corners of the original shape to the point where they are significantly rounded. In the case of a cross-shape pattern such as used with an IR filter array, for instance, the center of the structure is enlarged, while the width of the arms varies along their length. Moreover, the rounding of the corners and the widening of the center accumulates over multiple mask generations. Experiments done with 0,49 cm2 stencil masks confirmed that the quality of the mask pattern degrades from first to second generation. Therefore, the reproduction of high-frequency spatial information in multi-generational masks, in particular structures comprising comers and/or elbows of lines, is problematic.
The degradation of an IBP mask image is shown in FIG. 2. The graph shows the resist foot-print of five generations of IBP mask copies, where the resist footprint of the previous generation is used as the mask to print the next generation, according to a simulation calculation where the blur was chosen to be 70 nm FWHM. Clearly, the final, fifth generation mask pattern does not resemble the original mask pattern; rather, the initial cross pattern is considerably blotted.
It is an aim of the present invention to overcome the above-described problems with the production of patterns, in particular in the context of multi-generation reproduction of mask patterns.
This aim is met by a method for lithographic patterning of a plurality of identical structures onto a target substrate wherein, according to the invention, a template mask bearing a template structure pattern, comprising a plurality of identical template structures each consisting of a set of at least one structure element of circular shape, is used for lithographic patterning of the target substrate, wherein by means of a broad beam of energetic radiation the template mask is illuminated to form a structured beam and the template structure pattern is imaged onto the target substrate by means of the structured beam, the target substrate being positioned after the mask as seen in the optical path of the beam and comprising material sensitive to exposure to said energetic radiation, producing a pattern image on the target substrate, the pattern image thus produced comprising a plurality of identical target structures.
A template mask according to the invention, that is, a mask bearing a template structure pattern comprising a plurality of identical template structures each consisting of a set of at least one structure element of circular shape, is suitably produced by a method wherein starting from a primary mask bearing a primary structure pattern consisting of at least one structure element having a circular shape, a template mask bearing a template structure pattern, comprising a plurality of identical template structures each corresponding to the primary structure pattern, is produced using the primary mask to define the template structures, wherein
the production of the template mask is done in at least one lithographic mask structuring step wherein in each mask structuring step by means of a broad beam of energetic radiation a mask bearing a structure pattern is illuminated to form a structured beam and the structure pattern is imaged at least once onto an intermediate substrate by means of the structured beam, said intermediate substrate being positioned after the mask as seen in the optical path of the beam and comprising material sensitive to exposure to said energetic radiation, producing a pattern image on the intermediate substrate, and from the intermediate substrate thus patterned another mask having a structure pattern corresponding to the pattern image is produced,
the mask used in the first of said mask structuring steps is the primary mask, the mask used in each subsequent step, if present, is the mask produced from the previous step, and the mask produced from the last of said mask structuring steps is the template mask, and
in at least one of said mask structuring steps the pattern image imaged from the structure pattern is moved over the intermediate substrate to a number of different locations
According to the invention, the generation of the actually desired target structure pattern is deferred until the last step of lithographic patterning. In the preceding steps, namely, the mask structuring steps, only structure patterns consisting of circular apertures are imaged, which are far less problematic to reproduce.
The invention makes possible the accurate reproduction of the primary pattern structure for many generations of subsequent masks. In each generation of mask structuring, the pattern can be multiplied, and thus an enormous number of copies of the initial pattern can be produced. In the final mask which is then used as template mask according to the invention, the desired number of pattern copies is reached; this template mask is then used for patterning of the target substrates. In short, the invention makes it possible to fabricate array mask copies with enormous areas as compared to the initial area of a single target structure; for example, the production of an IR filter of 20xc3x9720 cm2 filled with target structures of about 1 xcexcm size seem achievable with state-of-the-art lithography technology in the short term.
In a preferred embodiment of the invention the final shape of the target structure is defined in the last step of target structuring. In this target patterning step, the pattern image is moved over the substrate through a sequence of image positions, the exposure with respect to the sequence of image positions superposing into a spatial distribution of exposure dose on the substrate, said distribution defining the structures thus patterned on the target substrate according to the exposure-dependent characteristic of said sensitive material.
Preferably, in this case the spatial distribution of exposure dose on the target substrate exceeds the specific minimum exposure dose of said sensitive material only within regions of the substrate field, said regions forming an exposure pattern comprising a plurality of identical structures.
In an advantageous aspect of the invention, the intensity of exposure is varied for each image position in a manner according to a prescribed set of intensities calculated from a representation of a pattern image to be produced on the target substrate. The variation of the exposure intensity of the image positions can be used, e.g., to correct for mutual influence of the effective dose exposure of neighboring structure elements. Preferably, the intensity of exposure may be controlled by varying the time of exposure for the respective image position. In one variant of this aspect, relating to the forming of 3D structures, the pattern image may be defined in terms of a spatial distribution of height of said exposure-sensitive material, and said set of intensities is then suitably calculated from said distribution of height using a predetermined functional dependence of the height on the exposure dose.
Moreover, it is of advantage in this case if the primary structure pattern as well as each of the template structures of the template mask consist of a single structure element of circular shape. Then the circular openings in the mask just define the positions of the target structures to be defined; due to the circular shape of the openings no information due to blurring can be lost during the multiplicative reproduction of the openings.
Suitably, the pattern image is moved through a discrete set of image positions in the target patterning step. This also offers a simple approach to correct for cross-exposure effects between different image positions, wherein the time during which the pattern is held at an image position is varied according to the exposure dose to be imparted to the respective image position.
In a further preferred embodiment of the invention, the energetic radiation comprises electrically charged particles and the lithographic patterning is done using a particle optical lithography system, and the pattern image is moved over the substrate by inclining the direction of the beam with respect to the optical axis of said lithography system by means of an electrostatic deflection means of said lithography system. This makes use of the high accuracy with which the image can be moved over the substrate due to the electrostatic control of the optical apparatus.
Advantageously the energetic radiation in the mask structuring and the target patterning steps comprises electrically charged particles and the lithographic patterning is done using a particle optical lithography system. In particular the energetic radiation may comprise ions, such as hydrogen or helium ions. In this case, it is further suitable if the direction of the beam is inclined by an electrostatic deflection means of the particle optical lithography system.
The invention can be used with a variety of applications, one of which is the patterning of resist layers by energetic radiation. In this case, the target substrate comprises a layer of resist material sensitive to exposure to an energetic radiation. For instance when using ionbeam radiation, there is a host of applications for spatially varying exposure to the ion radiation, such as converting a GaAs substrate into an insulating state by bombardement with, e.g., hydrogen or oxygen ions, doping of semiconductor materials, hardening of material against etching or abrasive attack, or influencing the refractive index by irradiation.
It should be noted that the invention is, of course, not limited to the production of mesh filter arrays. One further application is the production of a stencil mask with a pattern optimized to compensate for blur or, in the case of electrons, proximity effects, based on a template mask according to the invention having a structure pattern comprising at least one circular opening. In this case the target substrate to be patterned is a mask substrate of a mask used in ion beam lithography, such as IBP lithography or ion beam projection lithography.
A lithography system especially suitable for the invention is provided with a template mask as described above, that is, a template mask bearing a template structure pattern comprising a plurality of identical template structures each consisting of a set of at least one structure element of circular shape.
In a further preferred embodiment of the invention, in each mask structuring step, the pattern image is moved over the intermediate substrate to a number of locations arranged in a regular array.